1. Field of the Invention
The invention is concerned with a method of making thin-walled cupped articles from a sheet of polyolefin plastics material and with the cupped articles so made.
2. Description of the Prior Art
It is known from U.K. Specification No. 1,367,338 to make thin-walled cupped articles from a sheet of polyolefin plastics material such as polypropylene, polyethylene, copolymers of such material and similar plastics materials. The method comprises introducing a thin sheet of the polyolefin plastics material in the solid-phase state, i.e. below the crystalline melting point and above the glass transition point, between a forming plug and a mold having an end opening, moving the forming plug into the mold opening with the sheet to carry a bulged portion of the sheet into the mold, and introducing pressure fluid into the bulged portion of the sheet to form the article in the mold.
The above method is particularly suitable for making thin-walled cupped articles of polypropylene or poly(propylene/ethylene) and has met with considerable commercial success. Such articles, if unpigmented, are transparent or clear, especially those made from polypropylene having a melt flow of at least 5. One disadvantage of such articles, especially those made from polypropylene having a melt flow of at least 5, is that the impact strength is low. It is known that the brittleness of polypropylene is reduced by orientation and that low temperature properties of polypropylene can be improved by copolymerization with other olefins, especially polyethylene. Encyclopedia of Polymer Science and Technology, 1969, Vol. 11, page 610). The disadvantage of low impact strength of the articles formed by the above method has been partly overcome by using sheets of polyolefin plastic material prepared from mixtures of polypropylene and poly(propylene/ethylene). These improved articles are commercially available and usually contain about 20%w of the copolymer. Although such copolymers do have a beneficial effect on the impact strength of the articles, there is a need for increasing the low temperature impact strength still further if the articles are to be stored at deep-freeze temperatures. Moreover, articles made from blends of said copolymers with homopolymer have reduced transparency or clarity compared with articles made from propylene homopolymer. This is a disadvantage for many uses of the articles. There is, therefore, a need for a means of increasing further the impact strength of the articles which does not adversely effect, or not to such a large extent, the transparency or clarity thereof.
After development of the present invention, which involves the use of sheet made from blends of propylene polymers with low density polyethylene, the following references were found in a search for related prior art.
U.K. Pat. No. 1,005,333 to Avison discloses that the slip characteristics of oriented polypropylene film can be improved by employing a blend of polypropylene with from 2% to 15% of polyethylene of 0.90-0.95 g/cc density and 0.5-12 melt index.
U.K. Pat. No. 1,139,887 to Union Carbide discloses that film produced from blends of polypropylene with 3 to 15 phr of polyethylene of 0.88 to 0.93 g/cc density and melt index in excess of 25 have better low temperature impact strength than polypropylene homopolymer film, as well as high gloss and low haze.
French Pat. No. 1,562,860 to Ethylene Plastique discloses that films produced from blends of polypropylene with 5-30% of polyethylene of density below 0.94, drawn below the crystalline melt temperature of polypropylene, have better mechanical properties than films correspondingly produced from polypropylene.